2 Marks Questions

Question 12 Marks

Calculate the amount of heat radiated per second by a body of surface area 12cm² kept in thermal equilibrium in a room at temperature 20^°C. The emissivity of the surface = 0.80 and $\sigma=6.0\times10^{-8}\text{Wm}^{-2}\text{K}^{-4}.$

Answer

$\text{A}=12\text{cm}^2=12\times10^{-4}\text{m}62$
$\text{T}=20^\circ\text{C}=293\text{K}$
$\text{e}=0.8$
$\sigma=6\times10^{-8}\text{w/m}^2\text{-k}^4$
$\frac{\text{Q}}{\text{t}}=\text{Ae}\sigma\text{T}^4=12\times10^{-4}\times0.8\times610^{-8}(293)^4$
$=4.245\times10^{-12}$
$=0.4245\approx0.42$

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Question 22 Marks

A copper sphere is suspended in an evacuated chamber maintained at 300K. The sphere is maintained at a constant temperature of 500K by heating it electrically. A total of 210W of electric power is needed to do it. When the surface of the copper sphere is completely blackened, 700W is needed to maintain the same temperature of the sphere. Calculate the emissivity of copper.

Answer

$\text{Q}=\text{e}\sigma\text{A}\big(\text{T}_0^4-\text{T}_1^4\big)$
For any body, $210=\text{eA}\sigma\big[(500)^4-(300)^4\big]$
For black body, $700=1\times\text{A}\sigma\big[(500)^4-(300)^4\big]$
Dividing $\frac{210}{700}=\frac{\text{e}}{1}\Rightarrow\text{e}=0.3$

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Question 32 Marks

On a hot summer day we want to cool our room by opening the refrigerator door and closing all the windows and doors. Will the process work ?

Answer

No. When the door of the refrigerator is left open in a closed room, the heat given out by the refrigerator to the room will be more than that taken from the room.
Therefore, instead of decreasing, the temperature of the room will increase at a slower rate.

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Question 42 Marks

Why is a white dress more comfortable than a dark dress in summer?

Answer

A white colour dress reflects almost all the radiations falling on it. So, it does not absorb any heat from the sunlight and we feel more comfortable in it.
On the other hand, a dark colour dress absorbs maximum radiation falling on it. So, we feel hot in a dark coloured dress during summers.

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Question 52 Marks

Does a body at 20^°C radiate in a room, where the room temperature is 30^°C? If yes, why does its temperature not fall further?

Answer

Yes, the body will radiate. However, its temperature will not fall down with time because as the temperature of the surroundings is greater than the temperature of the body.
So, its rate of absorption will be greater than its rate of emission.

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Question 62 Marks

On a cold winter night you are asked to sit on a chair. Would you like to choose a metal chair or a wooden chair? Both are kept in the same lawn and are at the same temperature.

Answer

We will prefer to seat on a wooden chair because as the conductivity of wood is poorer than that of metal, heat flow from our body to the chair will be less in case of a wooden chair.

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Question 72 Marks

An ordinary electric fan does not cool the air, still it gives comfort in summer. Explain.

Answer

An ordinary electric fan does not cool the air, still it gives comfort in summer because it circulates the air present in the room. Due to this, evaporation takes place and we feel cooler.

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